A microscopic description of the effect of hydration on the behavior of extraframework cations in zeoliteNa+−mordenite is reported. Energy minimization techniques, combined with appropriate interatomic potentialsto describe the potential energy surface of this complex system, have been used to determine the site selectivityof both cations and water molecules as a function of the hydration level. We have thus shown that the positionsof the cations in the main channels are substantially perturbed upon the sorption of water molecules whereasthose of the cations located in the small side channels are only slightly shifted. This modeling has beensuccessfully compared with experimental data obtained by dielectric relaxation spectroscopy.